Mobile device cases are now widely used following the explosion in use of smart phone and tablet devices. The applicant is currently producing a case which is described in WO 2015/101771. The case has an outer material of TPU which has a Shore Durometer hardness between 90A to 95A and an inner impact absorbing and damping material which is sold under the trade name Flexshock®. The impact absorbing material may, for example, be TPE, viscoelastic TPE, Poron or soft TPU and should preferably have a Shore Durometer hardness between 30A and 60A.
Such a case is highly successful commercially as the two-layered structure is aesthetically pleasing and provides good impact protection.
WO 2015/101771 contemplates a case in which a rigid polymer is used as an outer layer creating a three-layer structure. Other cases also use such a hard outer layer. The advantage of a hard outer layer is that it disperses the impact. The softer inner layer can then absorb this dispersed impact. However, an outer layer is prone to scratches and cracks. The hard outer material does not generally absorb energy and is generally provided just to provide a structural surface for the case.
Other cases are known such as the Otterbox Defender with an outer soft material and an inner hard material. The inner hard material frames the device and provides a rigid shell for the case while the outer softer material provides absorption of impact energy. However, any impact energy which passes through the soft material and reaches the hard material is transmitted to the device. The hard inner material may also scratch the device.
Obviously impact protection can be enhanced by increasing the thickness of at least one of the layers. However, thin and lightweight cases are far more desirable as they are more aesthetically pleasing, more convenient for a user to carry and have less negative effect on the radiofrequency performance of the device.